U.S. patent application number 10/430480 was filed with the patent office on 2003-12-04 for system for encapsulating ethernet frames over very high speed digital subscriber lines.
This patent application is currently assigned to 3COM CORPORATION. Invention is credited to Hurwitz, Alon, Lewin, Amit, Poddobny, Yuri, Yaron, Opher.
Application Number | 20030223446 10/430480 |
Document ID | / |
Family ID | 27734209 |
Filed Date | 2003-12-04 |
United States Patent
Application |
20030223446 |
Kind Code |
A1 |
Lewin, Amit ; et
al. |
December 4, 2003 |
System for encapsulating ethernet frames over very high speed
digital subscriber lines
Abstract
An apparatus for and method of encapsulating Ethernet frame data
in Very high speed Digital Subscriber Line (VDSL) frames. The VDSL
frames are transmitted over a point to point VDSL link where they
are subsequently extracted and forwarded as standard Ethernet
frames. The VDSL facility transport system comprises an Ethernet to
VDSL Consumer Premises Equipment (CPE) coupled to a DSL Access
Multiplexor (DSLAM) over a VDSL transport facility. The Ethernet to
VDSL CPE functions to receive a 10BaseT Ethernet signal and
encapsulate the Ethernet frame into a VDSL frame for transmission
over the VDSL facility. The DSLAM is adapted to receive VDSL
frames, extract Ethernet frames therefrom and generate and output a
standard Ethernet signal. Ethernet frames are encapsulated within
VDSL frames and transmitted on the wire pair without regard to the
state of the SOC signals. This overcomes the problems associated
with syncing the transmission of the Ethernet data with the SOC
signals. The present invention also provides a method of providing
the receiving station an indication of the start of a VDSL frame. A
preamble having certain desirable characteristics such as good
autocorrelation properties, is used by the receiving station to
identify the start of a VDSL frame. To further ensure that a
detected start of frame is valid, the length field of the VDSL
frame is examined for a legal length value.
Inventors: |
Lewin, Amit; (Tel Aviv,
IL) ; Yaron, Opher; (Tel Aviv, IL) ; Hurwitz,
Alon; (Ramat Gan, IL) ; Poddobny, Yuri;
(Herzelia, IL) |
Correspondence
Address: |
Edward K. Runyan
McDonnell Boehnen Hulbert & Berghoff
32nd Floor
300 S. Wacker Drive
Chicago
IL
60606
US
|
Assignee: |
3COM CORPORATION
|
Family ID: |
27734209 |
Appl. No.: |
10/430480 |
Filed: |
May 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10430480 |
May 6, 2003 |
|
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09318887 |
May 26, 1999 |
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6608834 |
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Current U.S.
Class: |
370/419 ;
375/220 |
Current CPC
Class: |
H04L 12/40136 20130101;
H04L 12/4633 20130101; H04L 27/2662 20130101; H04L 12/2881
20130101; H04L 12/2856 20130101; H04L 12/413 20130101 |
Class at
Publication: |
370/419 ;
375/220 |
International
Class: |
H04L 012/56 |
Claims
What is claimed is:
1. A method of transporting Ethernet frames over a Very high speed
Digital Subscriber Line (VDSL) transport facility coupling a first
communication device and a second communication device, said method
comprising the steps of: receiving an input Ethernet frame data on
said first communication device from a first Ethernet compatible
communication device coupled thereto; encapsulating said received
Ethernet frame within a VDSL frame; inserting a preamble and length
field in said VDSL frame; transmitting said VDSL frame over said
VDSL transport facility; receiving VDSL frame data on said second
communication device; extracting said Ethernet frame from said
received VDSL frame and generating an output Ethernet frame
therefrom; and transmitting said output Ethernet frame to a second
Ethernet compatible communication device coupled to said second
communication device.
2. The method according to claim 1, wherein said step of
encapsulating comprises the step of stripping off the preamble and
start of frame fields from said Ethernet frame before placing said
Ethernet frame in said VDSL frame.
3. The method according to claim 1, wherein said length field
inserted in said VDSL frame represents the length of said Ethernet
frame without the Ethernet preamble and Ethernet start of frame
fields.
4. The method according to claim 1, wherein said step of generating
comprises the step of adding Ethernet preamble and Ethernet start
of frame fields to said extracted Ethernet frame.
5. A method of transporting Ethernet frames over a Very high speed
Digital Subscriber Line (VDSL) transport facility connecting a
first communication device and a second communication device, said
method comprising the steps of: receiving an input Ethernet frame
data on said first communication device from a first Ethernet
compatible communication device coupled thereto; generating a
multibyte preamble field for use by said second communication
device to identify the start of a VDSL frame; generating a length
field representing the length of said Ethernet frame to be
transmitted over said VDSL transport facility; generating a data
field comprising said Ethernet frame to be transmitted over said
VDSL transport facility; assembling said preamble field, length
field and said data field so as to generate a VDSL frame;
transmitting said VDSL frame over said VDSL transport facility;
receiving VDSL frame data on said second communication device;
extracting the Ethernet frame from said data field and generating
an output Ethernet frame therefrom; and transmitting said output
Ethernet frame to a second Ethernet compatible communication device
coupled to said second communication device.
6. The method according to claim 5, wherein said step of
encapsulating comprises the step of stripping off the preamble and
start of frame fields from said Ethernet frame before placing said
Ethernet frame in said VDSL frame.
7. The method according to claim 5, wherein said length field
inserted in said VDSL frame represents the length of said Ethernet
frame without the Ethernet preamble and Ethernet start of frame
fields.
8. The method according to claim 5, wherein said step of generating
comprises the step of adding Ethernet preamble and Ethernet start
of frame fields to said extracted Ethernet frame.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to data
communication systems and more particularly relates to a system for
transporting Ethernet frames over Very high speed Digital
Subscriber Lines (VDSL).
BACKGROUND OF THE INVENTION
[0002] There is a growing need among both individuals and
enterprises for access to a commonly available, cost effective
network that provides speedy, reliable services. There is high
demand for a high-speed data network, one with enough bandwidth to
enable complex two-way communications. Such an application is
possible today if, for example, access is available to a university
or a corporation with sufficient finances to build this type of
network. But for the average home computer user or small business,
access to high speed data networks is expensive or simply
impossible. Telephone companies are therefore eager to deliver
broadband services to meet this current explosion in demand.
[0003] One of the problems is that millions of personal computers
have found their place in the home market. Today, PCs can be found
in approximately 43% of all United States households and a full 50%
of United States teenagers own computers. Virtually every PC sold
today is equipped with a modem, enabling communication with the
outside world via commercial data networks and the Internet.
Currently, people use their PCs to send and receive e-mail, to
access online services, to participate in electronic commerce and
to browse the Internet. The popularity of the Internet is such that
there are an estimated 50 million users around the globe. These
figures indicate that in the past few years the personal computer
has fueled a dramatic increase in data communications and the
corresponding demands on the data networks that carry the
traffic.
[0004] The Internet serves as a good example of the increased
demands that have been placed on data networks. At first, Internet
access consisted of text only data transfers. Recently, with the
popularity of the World Wide Web (WWW) and the construction of
numerous sites with high quality content, coupled with the
development of Internet browsers such as Mosaic, Netscape Navigator
and Microsoft Explorer, the use of graphics, audio, video and text
has surged on the Internet. While graphics, audio and video make
for a much more interesting way to view information as opposed to
plain text, bandwidth consumption is significantly more. A simple
background picture with accompanying text requires approximately 10
times the bandwidth needed by text alone. Real-time audio and
streaming video typically need even more bandwidth. Because of the
increased requirement for bandwidth, activities such as browsing
home pages or downloading graphics, audio and video files can take
a frustratingly long period of time. Considering that the
multimedia rich World Wide Web accounts for more than one quarter
of all Internet traffic, it is easy to see why the demand for
bandwidth has outpaced the supply. In addition, the creative
community is pushing the envelope by offering audio and full motion
video on numerous sites to differentiate themselves from the
millions of other sites competing for maximum user hits.
[0005] As use of the Internet and online services continues to
spread, so does the use of more complex applications, such as
interactive video games, telecommuting, business to business
communications and videoconferenceing. These complex applications
place severe strains on data networks because of the intensive
bandwidth required to deliver data-rich transmissions. For example,
a telecommuter who requires computer aided design (CAD) software to
be transported over the data network requires a high-bandwidth data
pipeline because of the significant size of CAD files. Similarly, a
business to business transaction in which large database files
containing thousand of customer records are exchanged also consumes
large amounts of bandwidth. The same is true for users seeking
entertainment value from sites offering high quality video and
audio. The lack of available bandwidth in today's data networks is
the primary barrier preventing many applications from entering
mainstream use. Just as processing power limited the effectiveness
of early PCs, bandwidth constraints currently limit the
capabilities of today's modem user.
[0006] Most computer modem users access data through the standard
telephone network, known as plain old telephone service (POTS).
Equipped with today's speediest modems, dial up modems on a POTS
network can access data at a rate of 28.8, 33.6 or 56 Kbps. Dial up
modem transmission rates have increased significantly over the last
few years, but POTS throughput is ultimately limited to 64 Kbps.
While this rate may be acceptable for some limited applications
like e-mail, it is a serious bottleneck for more complex
transactions, such as telecommuting, videoconferenceing or
full-motion video viewing. To illustrate, full motion video
compressed, using the Motion Picture Entertainment Group (MPEG)-2
standard requires a data stream of approximately 6 Mbps, or roughly
208 times the throughput of a 28.8 Kbps modem. Thus, using today's
dial up modems, it would take more than 17 days to capture two
hours of video. As bandwidth demands continue to grow, providers
search for better ways to offer high speed data access. Further
complicating the problem is the need to deliver all these complex
services at an affordable price.
[0007] Today's most popular data access method is POTS. But as
discussed previously, POTS is limited when it comes to large data
transfers. An alternative to POTS currently available is Integrated
Services Digital Network (ISDN). In the past few years, ISDN has
gained momentum as a high-speed option to POTS. ISDN expands data
throughput to 64 or 128 Kbps, both from the network to the home and
from the home back to the network, and can be technically made
available throughout much of the United States and in many other
parts of the globe. Similar to POTS, ISDN is a dedicated service,
meaning that the user has sole access to the line preventing other
ISDN users from sharing the same bandwidth. ISDN is considered an
affordable alternative, and in general, ISDN is a much better
solution for applications such as Web browsing and basic
telecommuting. However, like POTS, it severely limits applications
such as telecommuting with CAD files and full-motion video viewing.
The latter requires roughly 39 times the throughput than that
provided by ISDN. Multichannel multipoint distribution service
(MMDS), a terrestrial microwave wireless delivery system, and
direct broadcast satellite (DBS), such as DirecTv and US Satellite
Broadcasting (USSB), are wireless networks. They both deliver high
bandwidth data steams to the home, referred to as downstream data,
but neither has a return channel through which data is sent back
over the network, referred to as upstream data. Although it is a
relatively affordable system to deploy for broadcast applications,
because it requires no cable wires to be laid, it falls short in
interactive access. In order to use a wireless system for something
as basic as e-mail, an alternate technology such as a telephone
line must be used for the upstream communications.
[0008] Another network delivery system is asymmetric digital
subscriber line (ADSL). Offering a downstream capacity of 6 Mbps or
more to the home, ADSL has the downstream capacity to handle the
most complex data transfers, such as full motion video, as well as
an upstream capacity of at least 500 Kbps. However, due to its
limitation of downstream bandwidth capacity, it essentially is a
single service platform. Also, since it has to overcome the
challenge of reusing several thousand feet of twisted pair wiring,
the electronics required at each end of the cable are complex, and
therefore currently very expensive.
[0009] Hybrid fiber coax (HFC), a network solution offered by
telephone and cable companies, is yet another option for delivering
high bandwidth to consumers known in the art. However, HFC has
limitations. HFC networks provide a downstream capacity of
approximately 30 Mbps, which can be shared by up to 500 users.
Upstream bandwidth is approximately 5 Mbps and also is shared. A
disadvantage with HFC is that shared bandwidth and limited upstream
capacity become serious bottlenecks when hundreds of users are
sending and receiving data on the network, with service
increasingly impaired as each user tries to access the network.
[0010] It is a current trend among telephone companies around the
world to include existing twisted pair copper loops in their next
generation broadband access networks. Hybrid Fiber Coax (HFC), a
shared access medium well suited to analog and digital broadcast,
comes up short when utilized to carry voice telephony, interactive
video and high speed data communications at the same time.
[0011] Fiber to the home (FTTH) is still prohibitively expensive in
the marketplace that is soon to be driven by competition rather
than costs. An alternative is a combination of fiber cables feeding
neighborhood Optical Network Units (ONUs) and last leg premises
connections by existing or new copper. This topology, which can be
called fiber to the neighborhood (FTTN), encompasses fiber to the
curb (FTTC) with short drops and fiber to the basement (FTTB),
serving tall buildings with vertical drops.
[0012] One of the enabling technologies for FTTN is very high rate
digital subscriber line (VDSL). VDSL is an emerging standard that
is currently undergoing discussion in ANSI and ETSI committees. The
system transmits high-speed data over short reaches of twisted pair
copper telephone lines, with a range of speeds depending upon
actual line length.
[0013] The VDSL standard as provided by the VDSL Draft
Specification being drafted by the ANSI T1E1.4 Technical
Subcommittee, provides guidelines for the transmitter and receiver
within the VDSL modem. The connection between the VDSL modem and
the CPE specifies a number of signals including TxData, RxData,
RxErr, TxCLK, RxCLK and TxSOC and RxSOC. The latter two signals,
i.e., TxSOC and RxSOC, provide an indication of the start of the
VDSL frame to the CPE for both transmission and reception.
[0014] It is intended that the SOC signal be used by the CPE to
synchronize the transmission and reception of the data to and from
VDSL modem. In the case of transporting Ethernet data over the VDSL
facility, a problem arises, however, when attempting to sync
Ethernet frames to VDSL frames. The problem with using these SOC
signals is that the VDSL frame is a fixed number of bytes, e.g.,
256 bytes, whereas the Ethernet frame may vary from 64 to 1518
bytes. Designing and implementing the circuitry, e.g., state
machines, timing and framing circuits, etc., to perform the
protocol matching, i.e., sync timing between Ethernet frames and
VDSL frames is very complicated and hence expensive to
implement.
[0015] It is desirable to have a means of transporting Ethernet
frame data over a VDSL transport facility that does not require the
complicated circuitry and state machines when utilizing the SOC
signals provided by the VDSL modem.
SUMMARY OF THE INVENTION
[0016] The present invention is an apparatus for and method of
encapsulating Ethernet frame data in Very high speed Digital
Subscriber Line (VDSL) frames. The VDSL frames are transmitted over
a point to point VDSL link where they are subsequently extracted
and forwarded as standard Ethernet frames.
[0017] A typical VDSL facility transport system comprises an
Ethernet to VDSL Consumer Premises Equipment (CPE) coupled to a DSL
Access Multiplexor (DSLAM) over a VDSL transport facility. The
DSLAM is typically located at the curb or before the `last mile` in
a subscriber loop. The Ethernet to VDSL CPE functions to receive a
10BaseT Ethernet signal and encapsulate the Ethernet frame into a
VDSL frame for transmission over the VDSL facility. Likewise, the
Ethernet to VDSL CPE also functions to receive a VDSL signal,
extract Ethernet frames therefrom and output them as standard
10BaseT Ethernet signals.
[0018] The DSLAM is adapted to receive VDSL frames, extract
Ethernet frames therefrom and generate and output a standard
Ethernet signal. Likewise, the DSLAM is also adapted to receive
standard Ethernet frames from an Ethernet input signal and
encapsulate them in VDSL frames for transmission over the VDSL
facility.
[0019] In accordance with the invention, the SOC signals provided
by the VDSL are not utilized in transmitting the Ethernet frame
data over the VDSL facility. Ethernet frames are encapsulated
within VDSL frames and transmitted on the wire pair without regard
to the state of the SOC signals. This overcomes the problems
associated with syncing the transmission of the Ethernet data with
the SOC signals.
[0020] The present invention also provides a method of providing
the receiving station an indication of the start of a VDSL frame. A
preamble having certain desirable characteristics such as good
autocorrelation properties is used by the receiving station to
identify the start of a VDSL frame. To further ensure that a
detected start of frame is valid, the length field of the VDSL
frame is examined.
[0021] The receiving station performs a check to determine whether
the preamble pattern detected is actually a preamble or is Ethernet
data within the payload of the VDSL frame. The length field
contains 16 bits allowing for 65,536 combinations but only
1518-64=1454 of them are valid since the payload of the VDSL frame
carries only Ethernet frame data which can only range from 64 to
1518 bytes. Thus, the length field is checked to further narrow the
chance of a wrong synchronization.
[0022] There is provided in accordance with the present invention a
method of transporting Ethernet frames over a Very high speed
Digital Subscriber Line (VDSL) transport facility coupling a first
communication device and a second communication device, the method
comprising the steps of receiving an input Ethernet frame data on
the first communication device from a first Ethernet compatible
communication device coupled thereto, encapsulating the received
Ethernet frame within a VDSL frame, inserting a preamble and length
field in the VDSL frame, transmitting the VDSL frame over the VDSL
transport facility, receiving VDSL frame data on the second
communication device, extracting the Ethernet frame from the
received VDSL frame and generating an output Ethernet frame
therefrom and transmitting the output Ethernet frame to a second
Ethernet compatible communication device coupled to the second
communication device.
[0023] The step of encapsulating comprises the step of stripping
off the preamble and start of frame fields from the Ethernet frame
before placing the Ethernet frame in the VDSL frame. The length
field inserted in the VDSL frame represents the length of the
Ethernet frame without the Ethernet preamble and Ethernet start of
frame fields. The step of generating comprises the step of adding
Ethernet preamble and Ethernet start of frame fields to the
extracted Ethernet frame.
[0024] There is also provided in accordance with the present
invention a method of transporting Ethernet frames over a Very high
speed Digital Subscriber Line (VDSL) transport facility connecting
a first communication device and a second communication device, the
method comprising the steps of receiving an input Ethernet frame
data on the first communication device from a first Ethernet
compatible communication device coupled thereto, generating a
multibyte preamble field for use by the second communication device
to identify the start of a VDSL frame, generating a length field
representing the length of the Ethernet frame to be transmitted
over the VDSL transport facility, generating a data field
comprising the Ethernet frame to be transmitted over the VDSL
transport facility, assembling the preamble field, length field and
the data field so as to generate a VDSL frame, transmitting the
VDSL frame over the VDSL transport facility, receiving VDSL frame
data on the second communication device, extracting the Ethernet
frame from the data field and generating an output Ethernet frame
therefrom and transmitting the output Ethernet frame to a second
Ethernet compatible communication device coupled to the second
communication device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention is herein described, by way of example only,
with reference to the accompanying drawings, wherein:
[0026] FIG. 1 is a block diagram illustrating an Ethernet to VDSL
CPE coupled to a DSLAM over a VDSL transport facility;
[0027] FIG. 2 is a block diagram illustrating the DSL Access
Multiplexor (DSLAM) in more detail;
[0028] FIG. 3 is a block diagram illustrating the Ethernet to VDSL
CPE in more detail;
[0029] FIG. 4 is a diagram illustrating the format of a standard
Ethernet frame;
[0030] FIG. 5 is a diagram illustrating the interframe gap between
two Ethernet frames;
[0031] FIG. 6 is a diagram illustrating the format of VDSL frames
that are transmitted over the VDSL facility;
[0032] FIG. 7 is a timing diagram illustrating the relationship
between the Rx_Err, SOC and VDSL data signals; and
[0033] FIG. 8 is a flow diagram illustrating the receiver
synchronization method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Notation Used Throughout
[0034] The following notation is used throughout this document.
1 Term Definition ADSL Asymmetric Digital Subscriber Line ANSI
American National Standards Institute CAD Computer Aided Design CAP
Carrierless Amplitude Modulation/Phase Modulation CPE Consumer
Premises Equipment CRC Cyclic Redundancy Check DBS Direct Broadcast
Satellite DC Direct Current DSL Digital Subscriber Loop DSLAM DSL
Access Multiplexor ETSI European Telecommunications Standards
Institute FCS Frame Check Sequence FDM Frequency Division
Multiplexing FEXT Far End Crosstalk FTTB Fiber to the Building FTTC
Fiber to the Curb FTTCab Fiber to the Cabinet FTTEx Fiber to the
Exchange FTTH Fiber to the Home FTTN Fiber to the Node HFC Hybrid
Fiber Coax IFG Interframe Gap ISDN Integrated Services Digital
Network MMDS Multichannel Multipoint Distribution Service MPEG
Motion Picture Entertainment Group NEXT Near End Crosstalk PC
Personal Computer POTS Plain Old Telephone Service QAM Quadrature
Amplitude Modulation QoS Quality of Service RF Radio Frequency RFI
Radio Frequency Interference SNMP Simple Network Management
Protocol SOC Start of Cell SOF Start of Frame USSB US Satellite
Broadcasting UTP Unshielded Twisted Pair VDSL Very High Speed
Digital Subscriber Line WWW World Wide Web
General Description
[0035] The present invention is an apparatus for and method of
encapsulating Ethernet frame data in Very high speed Digital
Subscriber Line (VDSL) frames. The VDSL frames are transmitted over
a point to point VDSL link where they are subsequently extracted
and forwarded as standard Ethernet frames. As used throughout this
document, the term VDSL frame is intended to denote a frame of data
having a variable length that is transmitted over the VDSL
transport facility. The length of the VDSL frame varies in
accordance with the length of the Ethernet frame encapsulated
within. The VDSL frame of the present invention bears no
relationship and should not be confused with the VDSL frame having
a fixed length of 405 bytes as described in the VDSL Draft
Specification published by the ANSI T1E1.4 Subcommittee.
[0036] A block diagram illustrating an Ethernet over VDSL transport
facility is shown in FIG. 1. The system, generally referenced 10,
comprises an Ethernet to VDSL Consumer Premises Equipment (CPE) 14
coupled to a DSL Access Multiplexor (DSLAM) 18 over a VDSL
transport facility 16. The Ethernet to VDSL CPE 14 functions to
receive a 10BaseT Ethernet signal 12 and encapsulate the Ethernet
frame into a VDSL frame for transmission over the VDSL facility 16.
Likewise, the Ethernet to VDSL CPE 14 also functions to receive a
VDSL signal and extract Ethernet frames therefrom for output as
standard 10BaseT Ethernet signals 12.
[0037] The DSLAM 18 is adapted to receive VDSL frames, extract
Ethernet frames therefrom and generate and output a standard
Ethernet signal. Likewise, the DSLAM 18 is also adapted to receive
standard Ethernet frames from an Ethernet input signal 20 and
encapsulate them in VDSL frames for transmission over the VDSL
facility 16.
[0038] The VDSL facility 16 may comprise any suitable transport
facility that is capable of transporting 10BaseT Ethernet data from
one point to another. Preferably the VDSL facility conforms to the
VDSL standard which is currently a draft specification being
formulated by the ANSI TIE1.4 Technical Subcommittee.
[0039] A transport facility suitable for use with the present
invention is the 10BaseS transport facility described in detail in
U.S. application Ser. No. 08/866,831 filed May 30, 1997, entitled
`Ethernet Transport Facility Over Digital Subscriber Lines,`
similarly assigned and incorporated herein by reference. A brief
description of this transmission system is given below.
[0040] The 10BaseS transport facility is capable of transmitting 10
Mbps Ethernet over existing copper infrastructure. The system
utilizes carrierless amplitude and phase modulation (CAP) which is
a version of suppressed carrier quadrature amplitude modulation
(QAM). QAM is the most commonly used form of high speed modulation
over voice telephone lines. The system also utilizes frequency
division multiplexing (FDM) to separate downstream channels from
upstream channels. In addition, FDM is also used to separate both
the downstream and the upstream channels from POTS and ISDN
signals. A substantial distance in frequency is maintained between
the lowest data channel and POTS frequencies to permit the use of
very simple and cost effective POTS splitters, which are actually
splitters/combiners. The upstream channel is placed above the
downstream channel in frequency. The downstream and upstream data
channels are separated in frequency from bands used for POTS and
ISDN, enabling service providers to overlay 10BaseS on existing
services.
[0041] The 10BaseS system combines copper access transmission
technology of Ethernet based services with Quality of Service (QoS)
guaranteed by the SRVP protocol and is capable of being fully
managed through an SNMP agent. The 10BaseS transport facility can
deliver symmetrical data at 12.96 Mbps (net 10 Mbps) over
unshielded twisted pair (UTP) telephone wires originally intended
for bandwidths of between 300 Hz and 3.4 KHz. QAM modulation and
blind equalization are used to achieve a high transmission speed
over existing copper infrastructure. In addition, the system is
able to cope with several sources of noise such as impulse noise,
e.g., POTS transients, radio frequency interference (RFI) noise and
crosstalk noise, i.e., both near end crosstalk (NEXT) and far end
crosstalk (FEXT). In terms of RF emissions, the system can operate
using underground cabling as well as overhead distribution
cabling.
[0042] The DSLAM 18 will now be described in more detail. A block
diagram illustrating the DSL Access Multiplexor (DSLAM) in more
detail is shown in FIG. 2. As described previously, the DSLAM 18
functions to encapsulate and extract Ethernet frames into and from
VDSL frames. The DSLAM typically is adapted to generate a plurality
of VDSL streams to be transmitted over a plurality of VDSL
facilities 30 via one or more VDSL transceivers 32 at the front
end. The DSLAM comprises a high speed Ethernet port at the back
end, an Ethernet switch 36, Ethernet encapsulation/extraction
circuitry 35 and a plurality of VDSL transceivers 32. The
transceiver 40 functions to receive, for example, a 10BaseT Fast
Ethernet signal 42 and provide bidirectional Fast Ethernet
communications.
[0043] In the Ethernet to VDSL direction, Ethernet frames are
received over the 100BaseT Fast Ethernet port 42 and are input to
the Fast Ethernet transceiver 40. The Fast Ethernet signals are
input to an Ethernet switch 36 capable of switching at Fast
Ethernet speeds. The GT48212 Switched Ethernet Controller
manufactured by Galileo Technology, San Jose, Calif. can be used to
construct the Ethernet switch of the present invention. The switch
36 is coupled via signal lines 34 to circuitry 35 that performs
Ethernet encapsulation and extraction. The Ethernet
encapsulation/extraction circuitry 35 functions to encapsulate the
Ethernet frame data from each of the channels output of the witch
36 into VDSL frames and forward them via signal lines 33 to the
VDSL transceiver 32 corresponding to that particular channel. The
VDSL transceivers 32 modulate the VDSL frame data and generate a
VDSL signal suitable for transmission over the twisted wire pairs
30. Note that the VDSL frames may be transmitted using the 10BaseS
transport facility described above.
[0044] In the VDSL to Ethernet direction, VDSL signals, e.g.,
10BaseS signals, are received by one or more VDSL transceivers 32
over the twisted pair wires 30. A VDSL modem suitable for use in
constructing the VDSL transceivers 32 of the present invention
comprises the BCM6010 VDSL Transceiver manufactured by Broadcom,
Irvine, Calif. or VDSL transceivers manufactured by Savan
Communications Ltd., Netanya, Israel.
[0045] Each VDSL transceiver 32 functions to demodulate the signal
received over the twisted pair wires 30 and output VDSL frames via
signal lines 33 to Ethernet encapsulation/extraction circuitry 35.
The Ethernet encapsulation/extraction circuitry 35 functions to
extract the Ethernet frame data encapsulated within the VDSL frame
and generate standard Ethernet frames, which are then input via
signal lines 34 to the Ethernet, switch 36. The switch forwards the
Ethernet frames to the transceiver 40 for transmission over the
100BaseT port 42.
[0046] The Ethernet to VDSL CPE unit will now be described in more
detail. A block diagram illustrating the Ethernet to VDSL CPE in
more detail is shown in FIG. 3. The Ethernet to VDSL CPE unit 14
comprises an Ethernet transceiver 50, Ethernet
encapsulation/extraction circuitry 52 and VDSL transceiver 54. The
Ethernet transceiver 50 is adapted to receive and transmit standard
10BaseT Ethernet signals 12. An Ethernet transceiver suitable for
use with the present invention comprises the LXT905 10BaseT
Ethernet transceiver manufactured by Level One Communications,
Inc., Sacramento, Calif.
[0047] The transceiver 50 communicates with the
encapsulation/extraction circuitry 52 via signal lines that
comprise Tx and Rx data lines and a plurality of Tx and Rx control
lines. The Ethernet encapsulation/extraction circuitry 52 performs
protocol conversion between Ethernet and VDSL frame formats. A VDSL
modem suitable for use in constructing the VDSL transceiver 54 of
the present invention comprises the BCM6010 VDSL Transceiver
manufactured by Broadcom, Irvine, Calif. or VDSL modems
manufactured by Savan Communications Ltd., Netanya, Israel.
[0048] In the Ethernet to VDSL direction, Ethernet frames are
received over the 10BaseT Ethernet port 12 and are input to the
Ethernet transceiver 50. The Ethernet signals are input, via Tx and
Rx data and control lines, to the Ethernet encapsulation/extraction
circuitry 52 which functions to encapsulate the Ethernet frame data
received over the Ethernet port 12 into VDSL frames. The VDSL
frames are then forwarded to the VDSL transceiver 54. The VDSL
transceiver 54 functions to modulate the VDSL frame data and
generate a VDSL signal suitable for transmission over the twisted
wire pair 16. Note that the VDSL frames may be transmitted using
the 10BaseS transport facility described above.
[0049] In the VDSL to Ethernet direction, VDSL signals, which may
comprise 10BaseS signals, are received by the VDSL transceiver 54
over the twisted pair wire 16. The VDSL transceiver 54 functions to
demodulate the signal received over the twisted pair wire 16 and
output VDSL frames to the Ethernet encapsulation/extraction
circuitry 52. The Ethernet encapsulation/extraction circuitry 52
functions to extract the Ethernet frame data encapsulated within
the VDSL frame and generate standard Ethernet frames which are then
forwarded to the Ethernet transceiver 50 for transmission over the
10BaseT port 12.
[0050] The VDSL transceiver 54 functions to provide the clocking
via TxCLK and RxCLK signals for both transmit and receive data
signals TxData, RxData. In addition, the transceiver 54 provides a
RxErr signal that is asserted when an error is detected in the
received data. An error condition may comprise a framing error,
loss of synchronization of the receive signal, etc. Further, the
transceiver 54 provides a Tx and Rx Start of Cell (SOC) signal,
TxSOC, RXSOC. The SOC signals, as defined in the VDSL draft
standard, are suitable for use in transporting ATM cell data over
VDSL but are suitable also for general use in synchronizing the
TxData signal input to the transceiver and the RxData output of the
transceiver. The Tx and Rx SOC signals provide a pulse at the
beginning of the VDSL frame. A VDSL frame comprises a fixed number
of bytes, e.g., 256, which has no relation to the number of bytes
in an Ethernet frame.
[0051] As discussed previously in the Background Section of this
document, the circuitry required is very complex to design to
synchronize Ethernet frames to the VDSL frames in accordance with
the SOC signals. The present invention overcomes this problem by
sending and receiving Ethernet frame data over VDSL asynchronously
with respect to the Tx or Rx SOC signals. The invention functions
to transmit the Ethernet frame data over the VDSL channel without
utilizing the Tx or Rx SOC signals. This eliminates any problems
associated with synchronizing the Ethernet data to the SOC data.
Problems include, for example, breaking up the Ethernet frame data
into multiple sections to fit within the smaller VDSL frames (when
the Ethernet frame exceeds 256 bytes) and subsequently regenerating
the Ethernet frame by assembling the multiple smaller sections.
[0052] A diagram illustrating the format of a standard Ethernet
frame is shown in FIG. 4. A standard Ethernet frame, generally
referenced 60, comprises a plurality of fields. The fields include
a 7 byte preamble 62 consisting of 0xAA characters, a one byte
Start of Frame (SOF) character 64 consisting of OXAB, a 6 byte
destination address 66, a 6 byte source address 68, 2 byte
type/length 70, 46 to 1500 byte data field 72 and a 4 byte Frame
Check Sequence 74 that comprises a CRC check. The type/length field
70 may comprise either type or length data, depending on the
variant of Ethernet used. The fields comprising the destination
address 66, source address 68, type/length 70, data 72 and FCS 74
are commonly referred to as the Ethernet frame. Note that the
Ethernet frame may comprise from 64 to 1518 bytes depending on the
size of the data field. Data shorter than 46 bytes is padded to a
minimum of 46 bytes.
[0053] In accordance with the 802.3 standard, Ethernet data is
transmitted using Manchester coding whereby an idle character is
transmitting using DC and a `0` and `1` characters are transmitted
having a transition half way through the symbol, the transition for
a `0` being opposite that for `1`.
[0054] A diagram illustrating the interframe gap between two
Ethernet frames is shown in FIG. 5. The Ethernet IEEE 802.3
standard provides for a minimum Interframe Gap (IFG) of 9.6 s
between frames to facilitate collision detection and avoidance. The
9.6 s IFG is equivalent to 12 bytes for 10 Mbps Ethernet. An
example is shown whereby two Ethernet frames 80, 82 are separated
by an IFG of 9.6 s. The IFG is removed by the CPE 14 and is not
transmitted over the VDSL facility. The IFG is inserted, however,
when transmitting Ethernet frames constructed from VDSL frame data
received over the VDSL facility.
[0055] A diagram illustrating the format of VDSL frames that are
transmitted over the VDSL facility (or 10BaseS facility) is shown
in FIG. 6. The VDSL frame, generally referenced 90, comprises a 5
byte preamble field 92, a 2 byte length field 94 and a data field
96. The preamble 92 comprises any suitable bit pattern that
facilitates reception, detection and synchronization of the VDSL
signal at the receiver. Preferably, the preamble is chosen to have
optimal autocorrelation properties. The preamble field is used by
the receiving station to identify a start of VDSL frame. Note that
this field should not be confused with the 7 byte preamble field 62
(FIG. 4) of the Ethernet frame itself consisting of OxAA
characters.
[0056] The length field 94 conveys to the receiving station the
number bytes in the data field that follows. The data field
comprises the encapsulated Ethernet frame that may have a length of
64 to 1518 bytes (excluding the preamble and SOF fields). The
entire VDSL frame 90 can have a length, including the preamble and
length fields, ranging from 71 to 1525 bytes. Note that if the IEEE
802.1Q standard is to be supported, the frame may be 4 bytes
longer.
[0057] It is important to note that in accordance with the present
invention, as described previously, the VDSL frame is transmitted
without the use of the Tx or Rx SOC signals provided by the VDSL
transceiver. In place of the SOC signals, the preamble performs the
role of providing a means for the receiver in the VDSL transceiver
to know when a VDSL frame begins. The length field allows the
receiver to know when the VDSL frame ends.
[0058] A timing diagram illustrating the relationship between the
RxErr, SOC and VDSL data signals is shown in FIG. 7. As described
previously, the RxErr signal (trace 100) is generated by the VDSL
transceiver when sync is lost or any other error occurs in the
receiver. The SOC signal (trace 102) is shown comprising a pulse to
signal the start of the VDSL frame within the transceiver. The SOC
signal, however, is not used by the apparatus of the invention. The
data (trace 104) shown comprises a sequence of VDSL frames each
consisting of a preamble, length and data fields with zeros
inserted during idle times. As shown, the transmission of the data
is completely independent from the SOC signal 102. As an example, a
sync occurs as indicated by the dotted portion 106 of the Rx_Err
trace 100. The data received during this time may contain one or
more errors.
[0059] As described above, the Ethernet frame data boundaries
received from the 10BaseT port 12 (FIG. 1) have no correlation with
the VDSL frames transmitted over the VDSL facility 16. The Rx_Err
indication at the receiving station, however, does relate to the
VDSL data frames. If a VDSL frame is encountered that has errors,
i.e., the Rx_Err signal is asserted, the frame can either be
forwarded or dropped in accordance with an Rx_Err policy, which may
be set by the user. If the policy is to drop frames, and the frame
received contained non-idle Ethernet frame data, data will be lost.
Note that theoretically, up to four Ethernet frames may reside
within a single VDSL frame width, assuming 64 byte minimum size
Ethernet frames and 256 byte VDSL frames (as specified by the VDSL
Draft Standard).
[0060] In order to regain synchronization, the receiving station
starts looking for the 5 byte pattern of the predefined preamble
code. Note, however, that the Ethernet frame data encapsulated
within the payload of the VDSL frame may contain the exact bit
sequence of the preamble code pattern. This would result in the
receiver regaining the wrong synchronization. The probability of
this happening is given by Equation 1 below. 1 Pr ( preamble error
) = ( 1 2 ) 40 10 - 12 ( 1 )
[0061] In accordance with the present invention, the receiving
station performs a check to determine whether the preamble pattern
detected is actually a preamble or is Ethernet data within the
payload of the VDSL frame. The length field contains 16 bits
allowing for 65,536 combinations but only 1518-64=1454 of them are
valid. Thus, the length field can be checked to further narrow the
chance of a wrong sync. A flow diagram illustrating the receiver
sync method is presented below.
[0062] A flow diagram illustrating the receiver synchronization
method of the present invention is shown in FIG. 8. The first step
is to hunt for the preamble (step 110). If the preamble pattern is
detected (step 112), the first two bytes following the preamble are
read in (step 114). This is the length field. Since the VDSL frame
payload only carries Ethernet data, the value of the length field
must be in the range of 64 to 1518 bytes. If the value of the
length is less than 64 or more than 1518 (step 116), then the
preamble bit pattern detected was not a preamble indicating the
start of a VDSL frame. The Rx_Err signal is asserted by the VDSL
transceiver (step 120) and a search of the preamble starts anew
(step 110). If the length is legal, the remainder of the VDSL frame
is read in (step 118).
[0063] Using this sync method further decreases the probability of
obtaining the wrong sync by a factor given below. 2 Pr ( length
error ) = ( 1518 - 64 ) ( 1 2 ) 16 0.022 ( 2 )
[0064] Multiplying this Pr(length error) factor by the Pr(preamble
error) yields an overall probability for wrong synchronization
given by Equation 3 below.
[0065] Pr(wrong sync)=Pr(preamble error) Pr(length error) 3 Pr (
wrong sync ) = Pr ( preamble error ) Pr ( length error ) = ( 1 2 )
40 0.022 2.2 .times. 10 - 14 ( 3 )
[0066] This results in a relatively low probability of wrong
synchronization. Even in the event a non preamble is detected due
to the bit pattern occurring in the payload of the VDSL frame, the
upper layers of the protocol stack, i.e., the transport layer, will
detect an error and cause a retransmission or other error recovery
scheme.
[0067] While the invention has been described with respect to a
limited number of embodiments, it will be appreciated that many
variations, modifications and other applications of the invention
may be made.
* * * * *